Add a setting to choose grouping colors

[nandriychuk]

Jack asked if the color of the treatment grouping variables could be changed to match the results-overtime treatment variable color legend. We need to add a setting for this so we can customize it for future Safety Explorers.

Test notes < click here!

• Change the Group Variable control to each setting and verify the colors remain consistent, i.e. the color of the first group column never changes, the color of the second group column never changes, etc.
• Add a new set of colors and verify the color palette updates.